Signing on to a Star Wars project is a massive commitment. Years out of one’s life. An intense period of preparation, then relocating to England for the duration of the shoot. One reason Abrams initially balked at taking on TFA was either be away from his family or uproot them and bring them along.
Getting sacked from a Star Wars film is probably as mentally devastating as losing a presidential election. Even more so if you’ve been a life long fan. Who in their right mind is going to want that level of stress knowing the axe is constantly hanging over your neck?